Electronic power systems typically consist of one or more power converters controlled by integrated circuits. The power converters receive an electrical input having an input magnitude, frequency, and/or phase at an input side and convert the electrical input to an electrical output at an output side having a magnitude, frequency, and/or phase. An isolated power converter includes an isolation transformer to electrically isolate the power converter output stage or secondary side from the input stage or primary side by arranging the isolation transformer between the primary side and the secondary side. Transformers can be characterized by a hysteresis curve of magnetic flux density; often called the B-H curve. The B-H curve has four quadrants. During operation of the power converter, if the flux density in the transformer is characterized by only the first quadrant of the B-H curve, the isolated power converter is classified as single-ended. If the flux density in the transformer is characterized by the first quadrant and the third quadrant of the B-H curve, the isolated power converter is classified as double-ended. In general, a double-ended power converter requires a smaller transformer core than a single-ended power converter and does not need an additional reset winding.
Typically, the output stage of the isolated power converter includes a diode rectifier circuit. These rectifiers can be a source of energy loss in a power converter. The diode rectifiers can be replaced with synchronous rectifiers implemented with semiconductor switches (e.g., a metal oxide field effect transistor or MOSFET) having a lower on-state resistance to improve efficiency of the rectifier. However, there can be drawbacks to using synchronous rectifiers because of the bidirectional current flowing capability of the semiconductor switches.
In order to achieve the minimum conduction resistor, the timing of the synchronous rectifiers of the secondary side is complementary with the timing of the primary switches of the primary side. During startup, the synchronous rectifiers may discharge energy stored in the output capacitor of the power converter if the output capacitor is pre-charged. This discharged energy can be transferred from the secondary side to the primary side through the synchronous rectifiers if there is a large enough decrease in the input voltage. If the decrease in the input voltage is large enough, a large reverse current can be transferred from the output to the input and can cause damage to internal circuit components. If multiple converters are connected in parallel, the synchronous rectifiers in different converters can form a loop where a circulating current flows; leading to a large power loss even the output load is very light. Additionally, reverse current flow to the input can increase the root mean square (RMS) value of current flowing in the primary stage and decrease the efficiency of the power converted during light load operation. The present inventors have recognized a need for improved performance of power converter circuits.